Rumors of new forces

By Sean Carroll | August 12, 2005 3:52 pm

Eric Adelberger, leader of the experimental gravity group at the University of Washington, left a comment in the discussion about new forces, which is worth elevating to the front page here:

Please don’t get too excited yet about rumors concerning the Eot-Wash test of the 1/r^2 law. We can exclude gravitational strength (|alpha|=1) Yukawa violations of the 1/r^2 law for lambda>80 microns at 95% confidence. It is true that we are seeing an anomaly at shorter length scales but we have to show first that the anomaly is not some experimental artifact. Then, if it holds up, we have to check if the anomaly is due to new fundamental physics or to some subtle electromagnetic effect that penetrates our conducting shield. We are now checking for experimental artifacts by making a small change to our apparatus that causes a big change in the Newtonian signal but should have essentially no effect on a short-range anomaly. Then we will replace our molybdenum detector ring with an aluminum one. This will reduce any signal from interactions coupled to mass, but will have little effect on subtle electromagnetic backgrounds. These experiments are tricky and measure very small forces. It takes time to get them right. We will not be able to say anything definite about the anomaly for several months at least.

I suppose we have to go along, although it’s hard to enforce levels of excitement. More importantly, it’s precisely because these experimenters are so careful that we’ll have every right to be very excited if they ultimately announce that they’ve really discovered a deviation from Newton’s force law!

While we’re waiting, here’s a great review article about tests of gravity at short scales. If you read it now, you’ll be all ready to understand new results as they come in.

  • Fyodor

    I’m delighted with this statement from Adelberger. I think that all major experimental/observational groups should have a rumor control department. To imagine that one establishes scientfic integrity by ignoring rumors is a serious error. I don’t think, in particular, that the WMAP people have done themselves any favors at all. Some people in fact are beginning to think that maybe there was something seriously wrong with the first batch of data! Better to have everything out in the open.

  • Dave

    Wow. Awesome. Wow.

  • Quantoken

    Eric’s caution and honesty certainly should be complimented. But regardless how much effort they put in, it still does not warrant that once they report the final result, it could be automatically accepted without any independent examination and verification.

    The point is this type of experiment are extremely difficult since the singal is just too weak. You can push the technology limit, but you can NOT push BEYOND technology limit. Not even a little bit.

    For now I can immediately question it on two issues. One, have they wraped the whole experiment instruments within an enclosure of superconductors? Probably they don’t. Nothing suggests they ever did. So the magnetic field of the earth must penetrate into the instruments. And we know metal parts moving in magnetic field would incure some induced EM forces, biasing the experiment outcome. It’s going to be weak but still many times stronger than the tiny gravitational force. How do you account for that effect?

    Two, The so called Casimir force. I do not personally believe the Casimir experiments were credible enough to show that indeed it exists. But if you do believe Casimir effect is real, then you need to account for it. Since Casimir force is inverse proportional to the 4th power of the distance, at small scale it quickly takes over and completely dorminate over the gravity force, making the later too small to be detectable. I suspect that at 100 micron, the theoretical value of Casimir force is already more than the gravity force. ButI have not done the calculation.


  • Alejandro Rivero

    Lubos has also put some care on following this rumour. According a posting this week, “the newest rumors (via A.S. of Stanford) say that the anomaly has disappeared once again”. But he did a decent coverage a couple months ago, and in the “fast comments” section an interesting remark was uploaded by a nickname “Lee Smolin”:

    It is intriguing to note that there are several apparent observational
    anomalies at the scale of the observed vacuum energy including:
    1) the MOND scale,
    2) the Pioneer anomaly,
    3) the anomalous low power in the lowest multipoles of the CMB,
    4) neutrino masses, and now perhaps this.
    Some or all of these are likely to go away, but it is nevertheless tempting to hypothesize that they are all signals of the same new physics.
    For more on this see…lin/ smolin3.pdf
    Lee Smolin | 06.07.05 – 1:21 pm | #

    I find specially interesting the point (4) because in Connes-Lott (et Coquereaux et al) models the masses are interpetable as inverse of distances, asociated in some way to a discrete “dimension” separating left and right spinors. As the biggest distance is the smallest mass, the neutrino distance could have a role.

  • Plato


    You know that they are following methods here, so they must be given credit.

    Yes I applaud the examination and continued update as well, Sean. You know people make statements about the extra dimensional ideas and just having such a method to recognition and application for science testability is a good example here.

  • Quantoken

    Gordon Chalmers on Aug 12th, 2005 at 3:33 pm said:
    “Maybe Eric you need a SAR apparatus, with 10^30 detectors.”

    I do not know what SAR apparatus is. But it looks like the kind of big science project that will bankrupt the economy of the whole uniberse, not just US government :-)

    10^30 accounts to 10^10 earth population, times 10^9, which is the upper limit of possible alien civilizations, times 1 billion US$, which is 10^11 cents, assuming 1 billion dollars is the upper limit of the how afluence aliens can be, earthlines are certainly poorer :-)


  • Jim Greenfield

    If ALL the earth’s mass was REALLY at the Center of Gravity, there would be a difference in the arcs of passing asteroids etc. The explanations of tidal forces doesn’t wash. Newton’s r^2 will work at large separations, as all the mass does seem to act in that direction(s), but if we live in a valley, we cannot ignore the mountains to the sides and above; their mass acts SIDEWAYS still, and it is not just that mass which is closer to earth center which is acting with us.
    At large distances it may not be apparent (make much difference) whether a mass was a point object (dense), or defined only by its center of mass (gravity). But if pulsars are two close massive objects revolving about a common center of gravity (mass), things may look a little wierd– such as to where is the emitted emr gravitationally affected as it leaves the system??

    Jim Greenfield

  • Anna

    I was checking the Fermilab colloquium schedule for this fall

    ( — sorry, I don’t know how to make url links on the blog)

    and saw that Eric Adelberger will be giving a talk on November 9.

    I thought I’d mention it here in case people are in the area and want to hear about these experiments first hand.
    (Access to Fermilab does not require a pass these days. For the latest see


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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